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PROTEINS INVOLVED IN NADP+/H METABOLISM AFFECT THE INDUCTION STATE OF THE soxRS REGULON IN Escherichia coli. María Victoria Humbert, Gustavo Schlaen, Néstor Carrillo and Adriana Krapp IBR, Area Biología Molecular. Facultad de Cs. Bioquímicas y Farmacéuticas. UNR Bacteria have evolved sophisticated molecular mechanisms to monitor oxidant levels and to activate antioxidant defence genes in response to specific signals. Two transcription regulators serve in Escherichia coli as redox sensing proteins under oxidative stress: SoxR and OxyR (both produced constitutively and activated upon exposure to superoxide- and hydrogen peroxide-generating agents, respectively). These transcription factors become active when they are oxidized: the [2Fe-2S] clusters in SoxR and two cysteine residues in OxyR. During aerobic growth the S-Fe centres of SoxR dimer are maintained in the inactive reduced form by a reducing activity dependent on NADPH and dedicated protein/s, although the entire mechanism remains unclear. When oxidized SoxR, activates the transcription of the soxS gene. The SoxS protein induces transcription of the soxRS regulon, whose products act collectively to avoid and repair oxidative damage. Superoxide and NADPH/NADP+ appear to be specific signals. We studied the response of the soxRS regulon under overexpression of proteins involved in reactions with NADPH in the presence or in the absense of superoxide. The overexpression of the own malic enzyme and plant ferredoxin in E. coli changed the activation state of SoxR. These proteins participate in reactions that involve NADPH/NADP+ either as substrate or partner. High levels of pea ferredoxin in E. coli led to a rise of SoxS in the absense of stress, whereas the increase of malic enzyme resulted in down-regulation of SoxS after oxidative stress. These changes were observed by measuring b-galactosidase activity from a transcriptional soxS fusion and by western blot. Overexpression of proteins that changes the NADPH pool affects SoxS expression, either in the ground state or in the presence of compounds that generate superoxide, indicating that the levels of NADPH act as a sole signal to trigger the soxRS response. These observations are consistent also with oxidation of the SoxR [2Fe-2S] clusters probably due to interferences with the NADPH pathways of responsible protein(s) that maintain SoxR in the reduced form.